Mass Air Flow (MAF) can be measured using hotwire or hotfilm anemometer type sensors. These types of sensors are used in engine control systems for gasoline engines and diesel engines. MAF measurements are used to control the proportion of fuel to air in the engine. MAF sensors convert air flowing past a heated sensing element into an electronic signal. The strength of the signal is determined by the energy needed to keep the element at a constant temperature above the incoming ambient air temperature. As the volume and density (mass) of airflow across the heated element changes, the temperature of the element is adjusted to maintain the desired temperature of the heating element. The varying current flow parallels the particular characteristics of the incoming air (hot, cold, dry, humid, high/low pressure). A control module monitors the changes in current to determine air mass and to calculate precise fuel requirements.
During transient engine operations, MAF sensor reading delays, or phase shifts can adversely affect control of the air fuel ratio, engine smoke control systems, and exhaust gas recirculation (EGR) systems. Many attempts have been made to overcome the transient delay of MAF sensor readings. One approach applies digital averaging software and filtering functions to artificially shift MAF sensor signals. Another method applies a manifold volume filling model.
These methods were developed to correct MAF sensor over predictions of fresh air mass per cylinder. The methods do not correct severe under predictions of fresh air mass per cylinder. Under predictions can occur during transient operations of the engine. An under prediction of air flow can severely penalize the vehicles driveability. The methods also fail to take into account engine speed change effects. The methods are not applicable to initial vehicle launch conditions of a diesel engine with a turbocharger where manifold pressure changes are small due to turbo lag, but rapid changes in engine speed are present.
Speed-density calculations or multi-zoned Dyna-Air algorithms are also used instead of MAF sensors. These methods can be complicated and require the availability of large sets of test data.